Fog

World of Earth Science
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Fog

If the atmospheric visibility near the earth's surface is reduced to 0.62 mi (1 km) or less due to floating water droplets in the air, it is called fog. Fog can form in two ways: either by cooling the air to its dew point (e.g., radiation fog, advection fog, upslope fog), or by evaporation and mixing, when moisture is added to the air by evaporation, and then it is mixed with drier air (e.g., evaporation fog, frontal fog). Other types of fog include ice fog (a fog of suspended ice crystals , frequently forming in Arctic locations), acid fog (fog forming in polluted air, and turning acidic due to oxides of sulfur or nitrogen), or smog (fog consisting of water and smoke particles). While any type of fog can be hazardous because of its effects on atmospheric visibility for ground and air transportation, acid fog and smog can pose additional risk to human health, causing eye irritations or respiratory problems.

Radiation fog (or ground fog) occurs at night, when radiational cooling of the earth's surface cools the shallow moist air layer near the ground to its dew point or below, so the moisture in the air condenses into fog droplets. It occurs under calm weather conditions, when light wind , or no wind at all is present, since a strong wind would mix the lower-level cold air with the higher-level dry air, thus preventing the air at the bottom from becoming saturated enough to create fog. The presence of clouds at night can also prevent fog formation of this type, because they trap the earth's heat, not allowing the cooling of the air for condensation . Radiation fog often forms in late fall and winter nights, especially in lower areas, because cold and heavy air moves downhill, and gathers in valleys. Accordingly, radiation fog is also called valley fog. In the morning it usually dissipates or "burns off" when the Sun's heat warms the ground and air.

Advection fog forms when warm, moist air horizontally moves (which is called advection) over a cold surface, which cools the air to its dew point. Advection fog can form any time, and can be very persistent. It is common along coastlines where moist air moves from over the water to over the land, or when an air mass moves over a cold surface (e.g., snow), and the moisture in the air condenses into fog as the surface cools it. Advectionradiation fog forms when warm, moist air moves over a cold surface, which is cold as a result of radiation cooling. When warm, humid air moves over cold water, it is called sea fog.

Upslope fog forms in higher areas, where a moist air mass is forced to move up along a mountain. While the air mass is moving up the slope, it is cooled beyond its dew point and produces fog. It requires a fast wind, and warm and humid conditions at the surface. Unlike radiation fog, this type of fog dissipates when no more wind is available, and it can also form under cloudy skies. Upslope fog is usually dense, and extends to high altitudes.

Evaporation fog forms by the mixing of two unsaturated air masses. Steam fog is a type of evaporation fog, which appears when cold, dry air moves over warm water or warm, moist land. When some of the water evaporates into low air layers, and the warm water warms the air, the air rises, mixes with colder air, cools, and condenses some of its water vapor. Over oceans , it is referred to as sea smoke. Examples of cold air over warm water occur over swimming pools or hot tubs, where steam fog easily forms. It is common, especially in the fall season, when winds are getting colder but the water is only slowly turning colder.

Precipitation fog is a type of evaporation fog that happens when relatively warm rain or snow falls through cool, almost saturated air, and evaporation from the precipitation saturates the cool air. It can turn dense, persist for a long time, and may extend over large areas. Although it is mostly associated with warm fronts, it can occur with slow cold fronts or stationary fronts as well, hence the name frontal fog is also used.

See also Hydrologic cycle

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fog

fog1
/ fôg; fäg/
•
n.
a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface that obscures or restricts visibility (to a greater extent than mist; strictly, reducing visibility to below 1 km):
the collision occurred in thick fog. ∎ [in sing.]
an opaque mass of something in the atmosphere:
a whirling fog of dust. ∎ [in sing.] fig.
something that obscures and confuses a situation or someone's thought processes:
the origins of local government are lost in a fog of detail. ∎ Photog.
cloudiness that obscures the image on a developed negative or print.
•
v.
(fogged
, fog·ging
) [tr.]
1.
cause (a glass surface) to become covered with steam:
hot steam drifted about her, fogging up the window. ∎ [intr.]
(of a glass surface) become covered with steam:
the windshield was starting to fog up. ∎ fig.
bewilder or puzzle (someone):
she stared at him, confusion fogging her brain. ∎ fig.
make (an idea or situation) difficult to understand:
the government has been fogging the issue. ∎ Photog.
make (a film, negative, or print) obscure or cloudy.
2.
treat with something, esp. an insecticide, in the form of a spray:
Winnipeg stopped fogging for mosquitoes three years ago.PHRASES:in a fog
in a state of perplexity; unable to think clearly or understand something.fog2 •
n.
the grass that grows in a field after a crop of hay has been taken. ∎
long grass left standing in a pasture and used as winter grazing.

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fog

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

fog, aggregation of water droplets or ice crystals immediately above the surface of the earth (i.e., a cloud near the ground). A light or thin fog is usually called a mist. Fog may occur when the moisture content of the air is increased beyond the saturation point. For example, fog usually results from the evaporation of warm water into cold air, which occurs when cold air streams over a warm water surface (steam fog) or when a warm rain falls through a layer of cold air near the ground (frontal fog). Fog also occurs when the air is cooled below a critical temperature called the dew point. Fog may be caused by radiation of heat from the ground during a windless, cloudless cool night (radiation fog); by the flow of warm air over a cold land or water surface (advection fog); or by air ascending a slope and cooling by expansion (upslope fog). In all cases condensation of the excess moisture takes place on the microscopic dust particles (condensation nuclei) in the atmosphere. Fog commonly found in valleys and depressions in the morning, especially during autumn, is of the radiation type, which because of its shallow nature is dissipated by the sun's heat as the day progresses. The extensive fog banks frequently occurring along the coasts of Newfoundland and Labrador are of the advective type and, being generally quite deep, often persist for days at a time, hindering shipping and aviation activity. In arid areas where fog is common, water may be harvested from the fog by using so-called fog nets.

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fog

fog Condition of atmospheric obscurity near the ground surface, caused by the suspension of minute water droplets in conditions of near saturation of the air. The formation of fog is aided by any concentration of smoke particles, as these act as condensation nuclei, and may cause fog at levels of humidity below saturation point. Visibility is below 1 km. The cause of condensation of the water droplets may be radiation-cooling of the ground, advection of warm air over a cold ocean or cold ground, or conditions at a front. See also SMOG. Compare MIST.

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fog

fog A condition of atmospheric obscurity near the ground surface, caused by the suspension of minute water droplets in conditions of near saturation of the air. The formation of fog is aided by any concentration of smoke particles, as these act as condensation nuclei and may cause fog at levels of humidity below saturation point. Visibility is below 1 km. The cause of condensation of the water droplets may be radiation cooling of the ground, advection of warm air over a cold ocean or cold ground, or conditions at a front. See also smog.

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fog

fog (mist) Water vapour in the atmosphere that has condensed around particles of dust at or near the ground, as opposed to water vapour condensed as clouds. Fog forms when moist air is cooled below its dew point.

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